1. Field of the Invention
This invention relates to a scanning optical apparatus and a color image forming apparatus using the same, and particularly is suitable for a color image forming apparatus such as a color laser beam printer having for example, the electrophotographic process, a color digital copying machine or a multifunction printer using a plurality of scanning optical apparatuses and using a plurality of image bearing members (photosensitive drums) to form a color image.
2. Related Background Art
Heretofore, in the scanning optical apparatus of a laser beam printer, a digital copying machine, a multifunction printer or the like, a beam light-modulated in conformity with an image signal and emitted from light source means has been periodically deflected by a light deflector comprising, for example, a rotary polygon mirror, and has been converged into a spot shape on the surface of a photosensitive recording medium (photosensitive drum) by a lens system having an fθ characteristic, and the surface of the recording medium has been light-scanned to thereby effect image recording.
FIG. 17 of the accompanying drawings is a schematic view of the essential portions of a conventional scanning optical apparatus. In FIG. 17, a divergent beam emitted from light source means 91 is made into a substantially parallel beam or a convergent beam by a collimator lens 92, and this beam (the quantity of light) is shaped by an aperture stop 93 and enters a cylindrical lens 94 having refractive power only in a sub scanning direction. The beam having entered the cylindrical lens 94 in a scanning cross section emerges in its intact state, and in a sub scanning cross section is converged and imaged as a substantially linear image near the deflecting surface 95a of a light deflector 95 comprising a rotary polygon mirror.
The beam reflected and deflected by the deflecting surface 95a of the light deflector 95 is directed onto a photosensitive drum surface as a surface 97 to be scanned by a lens system (scanning optical means) 96 having an fθ characteristic through a turn-back mirror 98, and the light deflector 95 is rotated in the direction of arrow A to thereby light-scan the photosensitive drum surface 97 in the direction of arrow B (scanning direction) and effect the recording of image information.
Herein, the direction in which the beam is reflected and deflected (deflected and scanned) by the light deflector is defined as the scanning direction, and the direction orthogonal to the optical axis of the scanning optical means and the scanning direction is defined as the sub scanning direction.
As conventional color image forming apparatuses such as a color laser beam printer and a color digital copying machine, there have been proposed various ones having a plurality of image bearing members (photosensitive drums) corresponding to the respective colors (yellow:Y, magenta:M, cyan:C, and black:Bk) of an output image.
Here, the color image forming apparatus comprises a construction in which for example, a plurality of above-described conventional scanning optical apparatuses corresponding to the respective image bearing members are disposed, or a construction in which a plurality of scanning optical apparatuses which can scan a plurality of image bearing members at a time.
For example, in Japanese Patent Application Laid-Open No. 8-50385, photosensitive drums which are four image bearing members corresponding to respective colors, i.e., yellow, magenta, cyan and black, are disposed and a scanning optical apparatus is disposed for each photosensitive drum. Images of the respective colors are superimposed on a conveyance belt to thereby form a desired color image.
In Japanese Patent Application Laid-Open No. 6-18796, two scanning optical apparatuses each of which can scan two photosensitive drums at a time are disposed for photosensitive drums which are four image bearing members corresponding to the respective colors to thereby form a desired color image.
When a color image is to be formed by the use of a plurality of scanning optical apparatuses, the positions of spots (dots) imaged on the image bearing members by the respective scanning optical apparatuses must be made coincident relative to one another in a whole scanning area in both of the scanning direction and the sub scanning direction. That is, it becomes necessary that in the scanning direction, the intervals among the spots be uniform and in the sub scanning direction, the inclination or curvature of the scanning line or the line intervals be uniform.
Unless the relative coincidence of the positions of the spots is made, when images are superimposed on the conveyance belt, the quality of an output image is degraded as color misregistration. Accordingly, it is important that the scanning apparatus is uniform and that the positional relationship between a scanning optical apparatus and an image bearing member corresponding thereto is coincident in each apparatus.
For example, in the above-mentioned Japanese Patent Application Laid-Open No. 8-50385, a color image is formed by the use of four scanning optical apparatuses and four image bearing members corresponding thereto. Here, it is desirable that relative to a dot formed on an image bearing member by a scanning optical apparatus, dots formed by the three other scanning optical apparatuses corresponding to this dot be all uniform when they are superposed on the conveyance belt.
In an actual scanning optical apparatus, however, the deviation between the above-mentioned dot positions occurs due to the accuracy errors of optical parts, the single piece accuracy error of a mechanical part to which an optical part such as an optical box is assembled, the assemblage error of the optical part and the error of the relative position between the scanning optical apparatus and the image bearing member.
When all of the four scanning optical apparatuses have the same various errors, any dot position deviation will not be brought about, but yet usually they have different errors and therefore this will cause color misregistration. Therefore a color image obtained will be one having color misregistration in the scanning direction and the sub scanning direction.
Japanese Patent Application Laid-Open No. 2001-150722 discloses an invention in which color misregistration in the scanning direction is taken up as a problem.
The color misregistration in the sub scanning direction in an optical factor can be broadly divided into two factors, i.e., a scanning line inclination component and a curvature component. For example, in the scanning optical apparatus shown in FIG. 17, if distortion occurs in the optical surface of the scanning optical means 96 and there is a single piece accuracy error or a lens is mounted in a state in which it is tilted in the axial direction parallel to the optical axis due to an assemblage error relative to the optical box, the dot position in the sub scanning direction comes to have an inclination component in the scanning line as shown in FIG. 18.
Here, for the simplification of description, it is to be understood that a phenomenon is typified regarding the color misregistration components of two of the four colors.
Assuming in FIG. 18 of the accompanying drawings that of the two colors is cyan (C) and the other is magenta (M), scanning line inclination occurs to both of the two colors: It is because unevenness occurs to error components such as a single piece accuracy error and an assemblage error that in FIG. 18, the amounts of inclination of the two colors differ from each other. If both of cyan and magenta have the same error component, that is, have an error from their ideal state, but there is no unevenness in that state, they also have the same amount of inclination and therefore there can be obtained a color image free of color misregistration.
Actually, however, both error factors have unevenness and therefore, the unevenness causes color misregistration. In order to reduce the color misregistration, in the conventional scanning optical apparatus, an adjusting mechanism is provided and for example, the whole of each scanning optical apparatus is inclined to thereby effect adjustment so that the scanning line may be adjusted to an ideal position.
When there occurs such a scanning line inclination as shown in FIG. 18, the inclination is adjusted about an axis parallel to the optical axes, for example, of the scanning optical apparatuses for cyan and magenta, whereby finally the inclination components of the two colors are made coincident with each other as shown in FIG. 19 of the accompanying drawings to thereby correct the color misregistration.
Likewise, when the lens has a single piece accuracy error, or when the lens is mounted in a state in which it is tilted or shifted in the axial direction parallel to the main scanning direction due to the assemblage error relative to the optical box, the dot position in the sub scanning direction comes to have a curvature component in the scanning line, as shown in FIG. 20 of the accompanying drawings.
Assuming as in FIG. 18 that one of the two colors is cyan (C) and the other is magenta (M), scanning line curvature occurs to both of the two colors. It is because unevenness occurs to the error components such as the single piece accuracy error and the assemblage error that in FIG. 20, the amounts of curvature of the two colors differ from each other. If both of cyan and magenta have the same error component, that is, have an error from their ideal state, but there is no unevenness in that state, the two colors come to have the same amount of curvature and therefore there can be obtained a color image free of color misregistration.
Actually, however, each error factor has unevenness and therefore, the unevenness causes color misregistration. In order to reduce the color misregistration, in the conventional scanning optical apparatus, an adjusting mechanism is provided and for example, in Japanese Patent Application No. 2000-258713, the turn-back mirror of a scanning optical apparatus is curved to thereby effect the adjustment of the curvature of a scanning line. That is, when such curvature of the scanning line as shown in FIG. 20 occurs, mirrors disposed in the scanning optical apparatuses for cyan and magenta are curved to thereby effect adjustment.
In such an adjusting method, however, the sensitivity of the mirrors to the curvature of the scanning line is low and therefore it is necessary to give great stress to the mirrors and curve the mirrors themselves and as a result, problems arise in the assemblage accuracy, environmental fluctuations and the accuracy of the adjustment itself, and this is not good.
So, as another method of adjusting the curvature of the scanning line, it is conceived to make at least one of lenses constituting the scanning optical apparatus eccentric. For example, the scanning optical means 96 of FIG. 17 is rotated about an axis parallel to the scanning direction to thereby effect the adjustment of the curvature of the scanning line.
However, when for example, the curvature of the scanning line (cyan:C) before adjustment and the scanning line sensitivity when the scanning optical means 96 is rotated about an axis parallel to the scanning direction are such as shown in FIG. 21 of the accompanying drawings, an error component d after adjustment remains at an intermediate image height. If this error component d remains, when the scanning optical apparatus is combined, for example, with a scanning optical apparatus free of the curvature of the scanning line, an amount of color misregistration a will occur. To completely eliminate this error component, it is necessary that the curvature of the straight, line before adjustment in FIG. 21 and the scanning line sensitivity of the scanning optical means 96 be coincident with each other.
However, the single piece accuracy, the assemblage accuracy, etc. which are the occurrence factors of the curvature of the scanning line occur with unevenness and therefore, there is also the possibility of the curvature of the scanning line before adjustment occurring with a form and an amount differing from those in FIG. 21.
Accordingly, it is very difficult to make the curvature of the scanning line before adjustment and the scanning line sensitivity of the scanning optical means 96 coincident with each other and thus, the error component remains even if the adjustment of the curvature of the scanning line is effected. If this error component is uniform for the four colors, color misregistration will not be brought about, but as described above, the single piece accuracy, the assemblage accuracy, etc. which are the occurrence factors of the curvature of the scanning line occur with unevenness and thus, the error component does not become uniform but occurs with unevenness. Because of this error component, color misregistration in the sub scanning direction is caused in image formation.
Also, in a color image forming apparatus, registration detecting means for each color component to detect a color misregistration component near the conveyance belt is disposed to thereby correct the writing start position in the sub scanning direction. Each scanning optical apparatus transfers a registration mark formed on the image bearing member to the conveyance belt, and the registration detecting means detects the position of this registration mark, whereby the amount of color misregistration in the sub scanning direction is detected. Design is made such that the phase control of the polygon motor is effected correspondingly to each scanning optical apparatus so that the color misregistration at the registration detecting position may be corrected on the basis of this information, and the respective colors are superimposed one upon another.
However, when an attempt is made to correct, for example, the color misregistration of cyan and magenta in which the curvature of the scanning line occurs, if the registration detecting means is disposed at the center of the scanning area, the amount of color misregistration will become null in the central portion of the scanning area but the amount of color misregistration will increase toward the peripheral portion of the scanning area and finally, great color misregistration in the sub scanning direction which is an amount of color misregistration d will occur in the peripheral portion.
When the correction of the four colors is effected, greater color misregistration may occur in the peripheral portion. Also, if conversely to the above-described example, registration detecting means are disposed on the opposite end portions of the scanning area, as shown in FIG. 22 of the accompanying drawings, the amount of color misregistration will become null in the opposite end portions of the scanning area, but the amount of color misregistration will increase toward the central portion of the scanning area and finally, great color misregistration in the sub scanning direction which is an amount of color misregistration d will occur. When the correction of the four colors is thus effected, greater color misregistration may occur in the central portion.